I have a 20 inch M16A4(ish) rifle that I keep with iron sights to practice irons and to have a no batteries needed rifle.

I had decided to do the USMC 300 yard zero and zero'd at the near distance of 36 yards.

However I put my rear sight on Z for zeroing since what I read online says to do that. I recently read the USMC manual for zeroing online and it says to stay at 3/6.

So the question is, how far off am I? Assuming I zeroed good and "should" have been on at 300, how much did zeroing at Z mess that up?

EDIT: ammo is 55 grn ball.